Community Forums

Manke · 03-13-2012, 06:24 PM

Hello All,

I've got an '08 CC Inspire 36' with a 425 ISL. Just passed 18,000 miles. We bought it back in June with just under 14k miles.
I am pretty religious about setting the cruise at 61 mph. Over the past summer , we did 1800 miles towing my truck and RZR (8,500 lbs total toad) and got as bad as 6.3 and as good as 7.2. I figured this was acceptable given the load I was towing.
Since then, we've had a handful of trips towing a 16' enclosed trailer, with all gear weighing 4,000 lbs max. Towing pretty much flat grades from Burbank out to the desert, and I'm getting anywhere from 6.05 - 6.60 mpg's. Again, always doing 60-61 mph. It seems to be getting worse the more I use it, regardless of if I'm towing or not!
I've also had 3-4 local trips towing nothing, and still not getting over 7 mpg's.

I've read A LOT of people claiming 7.5 to high 8's in MPG's on these larger engine DP's, so what gives?
What can I do to improve my chances of bettering my fuel economy?
I bought this pig (lovely pig that it is) expecting to get 7.5 - 8.0 mpg's, and this low 6's is getting to me.
I'm probably due for a new air filter, and will change all the engine/tranny fluids in a few months, I changed the fuel filter 4 months ago, but what else could be wrong?
Is there something that could be wrong with the programmer or something?

Tire pressure is always within spec, cold reading, 105 rear and 120 front, give or take 5 psi, but I'm certainly not low on tire pressure....

Looking for any tips on ways to combat this if possible....
Thanks

Our Time · 03-13-2012, 06:40 PM

Interesting post. I have a 2008 ISL 400 HP that has averaged 8.4 over the last 22,000 miles. I run at 62-64 MPH towing a 3800# toad. I am turning 1500 RPM at that speed. I wonder if your rear end gearing may be your problem. What RPM you turning at 61?
The only change to our engine from stock is we added a Horton fan clutch which I believe increased the mileage by 1/2 MPG.
I have heard of people getting better mileage by increasing the speed to 65, but have not tried it myself.

Manke · 03-13-2012, 06:57 PM

Quote:

Originally Posted by Our Time

Interesting post. I have a 2008 ISL 400 HP that has averaged 8.4 over the last 22,000 miles. I run at 62-64 MPH towing a 3800# toad. I am turning 1500 RPM at that speed. I wonder if your rear end gearing may be your problem. What RPM you turning at 61?
The only change to our engine from stock is we added a Horton fan clutch which I believe increased the mileage by 1/2 MPG.

Thanks for the response Chuck. I am right at 1500 RPM at 61. When at 60-62, it seems to be right around 1450-1600 RPM's. I can't recall what it is at 64 or 65 MPH, I'd guess around 1700 or so.

What an approximate cost for that Horton fan clutch and for install?

Our Time · 03-13-2012, 07:14 PM

As I recall the Horton was about $850 installed by Cummins about 3 years ago. It dramatically reduced the noise, better mileage and acceleration. Other options (fans) are now available and may be cheaper. I assume you have a rear radiator? If so, maybe someone will chime in here with more info on the fan kits or you could do a search on the Monaco forum which is where I saw it discussed.

Cat320 · 03-13-2012, 07:52 PM

We get high 7s to low 8s running 62/63. Never heard of increasing speed for better MPG. The worst we got was in the mid 6s bucking a 25/30 mph wind. FWIW, we have a side radiator and only 9k on it.

Manke · 03-13-2012, 08:20 PM

I have side radiator as well. I just spoke with an old CC tech about another part I needed, and he hadn't heard of this Horton clutch, and said it may do great things, but he said my coach already had an upgraded fan clutch. So I'm not sure...
Does having an existing side radiator reduce the benefit of these Horton fan clutches?

I've got many years of experience on my Duramax and old 5th wheels, but very new to the Motorhome game and all these moving parts to learn about, so I didn't know what all to ask him about that topic.

ricere · 03-13-2012, 09:50 PM

The Horton clutch is only good for rear radiators, I would think. You already have a fan clutch on the side radiator. My coach has a side radiator and the fan is hydraulic driven and others are driven by the serpentine belt with an electronic/mechanical clutch.

Dutch Star Don · 03-13-2012, 10:30 PM

Manke......I have a 36' rear radiator Monaco Diplomat (28000 lbs). I do the mileage both manually and via my VMSpc. I've just recently started making engine corrections on my VMSpc to make it more accurate. On our 2000 mile trip to New Mexico last month, I got the VMSpc dialed in pretty close. If it said I had used 30 gallons, I took 30.8 or maybe 31. With all that said, I've been getting a lifetime mileage (43000 miles) of 8.4 mpg. I probaly only have about 5000 of those miles towing either my boat or now my truck....both are about 4500 lbs.

The VMSpc is a great tool to watch what your engine is doing. An increase from 56 to 59/60 is almost 1 mpg. An increas from 56 to 61/63 is almost 2 mpg. My coach is a rocket at 36' with the ISL and it killed me to go slow. We literally only passed two vehicles on the 2000 mile trip.

I changed my rear radiator fan to a new design for Monaco's and picked up about .5 mpg. On this last trip we drove really slow (56 mph) with our friends who are towing a new/large fifth wheel. Without towing and at 56 mph we averaged 9.9 mpg. We got killed on two really windy days that sucked the mileage down from what would have been 10.5 mpg.

Doing the time math and mileage math, driving 56 is marginally worth it, but we decided to see what would happen. Along with the slower speeds, I also coast on off ramps and no hard acceleration unless absolutely necessary.

I also run a fuel additive that is pricey when added to the cost of fuel. I usully don't mention it because many get upset over additives.

Finally, if you're looking to save some money on fuel, get the PenFed credit card. It pays 5% back on all fuel purchases, when purchased at the pump. They deduct the 5% when they bill you with your monthly total.

sc3283 · 03-13-2012, 11:54 PM

Manke, cruise control is NOT always the answer for better MPG. Every slight downhill, they release, then at the bottom of the hill they accel. You have lost all your momentum. I know you state pretty flat land....but something to think about on your next trip.
I have always been able to get better mpg using my foot as compared to a cruise. I always let the vehicle roll on the downhills(within reason, nothing in the dangerous speeds) and let that carry me partially up the next hill.
Driving several vehicles with onboard dash MPG monitors, this type of driving has always shown more mpg than with a cruise( yes I know on dash is a reference only and not factual). I also always hand calculate mpg and always fill the fuel tank to the top of the filler neck so I know tank is equally full with every fuel stop.

Our Time · 03-14-2012, 04:20 AM

Quote:

Originally Posted by Cat320

We get high 7s to low 8s running 62/63. Never heard of increasing speed for better MPG. The worst we got was in the mid 6s bucking a 25/30 mph wind. FWIW, we have a side radiator and only 9k on it.

Our coach shifts into 6th gear at about 58-60 MPH. Running at 60-61 caused the engine to lug at the shift point. Increasing the speed brought the mileage up almost 1 MPG. This was documented by a test drive with a Cummins tech with his computor connected to the engine. I wouldn't have believed it either!

jlfbatonrg · 03-14-2012, 07:44 AM

Restricted air flow (dirty or clogged air filter) will also reduce mpg. I replaced the stock air filter with an aFEPower long life air filter. This air filter is used by many long haul truckers, and doesn't require replacement, just cleaning about every 25000 miles. Some data from our Cummins. brand new srock air filter, air restiction meter reads 11 inches. after 15000 air restriction meter reads 20 inches. With the aFE air restriction meter when clean reads 0 inches, after 25000 miles air restriction meter reads 4 inches. MPG on the stock air filter and ~15000 miles averaged 9.0 mpg. MPG on the aFE aire filter after ~24000 miles averaged 9.5. All mpg readings are towing a 3700 lb Saturn Vue, running at 62 mph in overdrive with cruise control in all kinds of weather conditions (head wind has a significant effect on mpg). Engine rpms were 1650. On our Allison we have a torque converter lockup (overdrive) that engages at about 62 mph, and drops the engine rpm from 1950 to 1650. I find I get better fuel mileage if we are running in overdrive with the transmission in Economy mode. The best mpg I have ever recorded was 10.5 driving East from LA to FL. and the worst was 7.8 going into a 35 mph head wind crossing Kansas. Good luck.

Jim

Cat320 · 03-14-2012, 10:06 AM

Quote:

Originally Posted by Our Time

Our coach shifts into 6th gear at about 58-60 MPH. Running at 60-61 caused the engine to lug at the shift point. Increasing the speed brought the mileage up almost 1 MPG. This was documented by a test drive with a Cummins tech with his computor connected to the engine. I wouldn't have believed it either!

I'll check it out next trip...sounds like a deal, get there sooner and better mileage.

We had a Duramax for a number of years hauling a big 5th wheel...at 30,000 the mpg improved...won't know about this big Cummins for a few years.